It is known to provide an auxiliary chamber which is connected and disconnected with an engine cylinder during the compression or expansion period of the engine cycle in order to influence the compression or expansion process. EP0095252 proposes using such an auxiliary chamber to trap a variable fraction of the cylinder charge during compression in order to vary the effective compression ratio of the engine. Pending patent application PCT/GB2004/001018 proposes using an auxiliary chamber as a buffer chamber in an air hybrid engine vehicle to transfer compressed gases in two steps via the auxiliary chamber to a compressed gas reservoir when the engine is operating as a gas compressor thereby assisting deceleration of the vehicle, and to expand the compressed gases in two steps from the compressed gas reservoir via the auxiliary chamber when the engine is operated as a gas expander thereby assisting acceleration of the vehicle. Another pending patent application GB0405828.5 proposes using an auxiliary chamber to vary the compression temperature of the cylinder charge in order to achieve CAI/HCCI combustion and influence the auto-ignition timing.
In order to apply the above proposed concepts, it has hitherto been necessary to provide a dedicated auxiliary chamber and a dedicated control valve connecting with the engine cylinder at the appropriate timing in order to vary the internal gas transfer. Such a system however adds cost and complexity, and is difficult to install in the limited space of the combustion chamber of the engine.